Method and system to increase electric brake clamping force accuracy

ABSTRACT

The method and system for increasing accuracy of clamping force of electric aircraft carbon brakes, once braking has been commenced, provides a first pair of electric brake actuators with a range of low brake clamping force responsive to low brake clamping force commands, and a second pair of electric brake actuators with a range of high brake clamping force responsive to high brake clamping force commands. The first pair of electric brake actuators is actuated to apply a minimum residual braking force once wheel braking is commenced, and the second pair of electric brake actuators is actuated only when the commanded braking force is in the high range of brake clamping force.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/826,531,filed Jun. 29, 2010, which is application is a continuation ofapplication Ser. No. 12/039,603, filed Feb. 29, 2008, now U.S. Pat. No.7,789,469, which is a continuation of application Ser. No. 11/337,097,filed 19 Jan. 2006, now U.S. Pat. No. 7,410,224, which are incorporatedby reference herein.

BACKGROUND OF THE INVENTION

This invention relates to a method and system for increasing accuracy ofclamping force of electric brakes of aircraft, and more particularlyrelates to a method and system for increasing accuracy of clamping forceof electric aircraft carbon brakes providing greater accuracy for lowbrake clamping force commands by dedicating a portion of a plurality ofelectric brake actuators of each brake to low brake clamping forcecommands, without otherwise affecting normal braking.

Commercial aircraft commonly have landing gear with electricallyactuated brakes for wheels mounted to the wing and body of the aircraft.The electrically actuated brakes are typically carbon brakes including atorque plate and a carbon heat sink stack containing the frictionsurfaces that are clamped together by four electric brake actuators witha clamping brake force to cause a wheel to decrease its speed ofrotation. In such a conventional airplane carbon brake system, whenbraking is commanded, either by a pilot's actuation of a brake pedal orautomatic braking, it causes the friction surfaces of the carbon brakesto make contact, creating brake torque to slow down the rotational speedof the wheel, and through contact with the ground, the taxi speed ofairplane.

As is described in application Ser. No. 11/061,375, it is possible toreduce brake wear of electrically operated aircraft carbon brakes, oncebraking has been commenced, by maintaining a minimum light residualclamping brake force when braking is no longer commanded, such as when apilot stops pressing on a brake pedal, or otherwise during a commandedrelease of braking during automatic braking. During taxiing ofcommercial aircraft, particularly at low speeds, steering of theaircraft is typically controlled by braking, and an unequal distributionof brake energy due to inaccurate metering of brake clamping force canin some instances interfere with the directional stability of aircraft,particularly when a minimum light residual clamping brake force ismaintained during taxiing when braking is no longer commanded. Unequaldistribution of brake energy due to inaccurate metering of brakeclamping force can also result in damage to wheels and brakes fromexposure to excessively high temperatures. It has been found that it isnot possible with currently available electrical braking systems toachieve a brake clamping force accuracy required by current industrystandards for the Boeing 787, and to prevent unequal distribution ofbrake energy. What is therefore needed is a method and system forproviding greater sensitivity to brake commands, particularly at lowbrake clamping force levels, without otherwise affecting normal braking.The present invention satisfies this and other needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides for amethod and system for increasing accuracy of clamping force of electricaircraft carbon brakes, once braking has been commenced, by providing afirst portion of electric brake actuators of each brake with a range oflow brake clamping force responsive to low brake clamping forcecommands, and a second portion of electric brake actuators of each brakewith a range of high brake clamping force responsive to high brakeclamping force commands, and actuating the first portion of electricbrake actuators with a range of low brake clamping force when thecommanded braking force is in the low range of brake clamping force, andactuating the second portion of electric brake actuators with a range ofhigh brake clamping force when the commanded braking force is in thehigh range of brake clamping force. The method and system of theinvention provide electric brake actuation with greater accuracy andsensitivity to brake commands, particularly at low taxiing speedsrequiring low brake clamping force levels, without otherwise affectingnormal braking.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the preferredembodiments in conjunction with the accompanying drawings, whichillustrate, by way of example, the operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for controlling electricbrakes of an aircraft, according to the present invention.

FIG. 2 is a graph illustrating the application of brake clamping forcevs. commanded brake application according to the present invention.

FIG. 3 is a schematic diagram of a system for controlling first andsecond pairs of electric brake actuators of an electric brake of FIG. 1,according to the present invention.

FIG. 4 is a schematic diagram of the system for increasing brakeclamping force accuracy according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While it is possible to reduce brake wear of electrically operatedaircraft carbon brakes, once braking has been commenced, by maintaininga minimum light residual clamping brake force when braking is no longercommanded during taxiing of commercial aircraft, particularly at lowspeeds, unequal distribution of brake energy due to inaccuracy of brakeclamping force can interfere with the directional stability of aircraft,and can result in damage to wheels and brakes from exposure toexcessively high temperatures.

Referring to the drawings, which are provided for purposes ofillustration and by way of example, the present invention accordinglyprovides for a method and system for controlling electrically operatedaircraft brakes of an aircraft having a plurality of wheels and acorresponding plurality of wheel brakes for the plurality of wheels toincrease accuracy of clamping force of electric aircraft brakesproviding greater accuracy for low brake clamping force commands bydedicating a portion of a plurality of electric brake actuators of eachbrake to low brake clamping force commands, preventing an unequaldistribution of brake clamping force without otherwise affecting normalbraking, such as when the aircraft is taxiing.

As is illustrated in FIG. 1, according to the method and system of theinvention, the commanded initiation of braking of any of the pluralityof wheel brakes 10 of an aircraft, such as by actuation of brake pedal12 by a pilot, an autobrake system 14, or gear-up braking system 16, forexample, is monitored by a brake actuation controller 18, and a residualbrake clamping force is set to a predetermined minimum residual brakeclamping force by the brake actuation controller to keep the brakesengaged and provide a slight drag for the plurality of wheel brakesfollowing the commanded initiation of braking. The predetermined minimumresidual brake clamping force is typically set to about 1 to 0% of themaximum brake clamping force of the brake, and in a currently preferredaspect, is set to about 2 to 5 percent of the maximum brake clampingforce of the brake.

Once the residual minimum brake clamping force is engaged, it ismaintained for the plurality of wheel brakes despite a commanded releaseof braking, such as by actuation of brake pedal by a pilot or anautobrake system, for example, of any of the plurality of wheel brakes.The application of the predetermined minimum residual brake clampingforce is continued until one or more control logic conditions occurs, inresponse to which the application of the predetermined minimum residualbrake clamping force is discontinued. In a preferred aspect, thepredetermined minimum residual brake clamping force is discontinued bysetting the residual brake clamping force to a “full dump” orsubstantially zero clamping force, so that the residual brake clampingforce would continue to be a “full dump” or substantially zero clampingforce until the brakes are applied again in the next commandedinitiation of braking. As is illustrated in FIG. 2, application of thepredetermined minimum residual brake clamping force is maintained aftercommanded release of braking results in a light brake drag duringtaxiing of an airplane.

Referring to FIG. 1, wheel speed monitors 20 for the wheels of theaircraft provide the wheel speed of the landing gear to the brakeactuation controller, which determines the average wheel speed andcompares the average wheel speed with a wheel speed threshold. A primarycontrol logic condition under which the application of the predeterminedminimum residual brake clamping force is discontinued occurs when theaverage wheel speed is below the predetermined wheel speed threshold,which in one presently preferred aspect is a wheel speed in a range ofabout 2 knots to about 10 knots, for example, in order to ensure fullbrake release during towing/push-back.

Typically when an aircraft has left landing gear 11 a and right landinggear 11 b, the average wheel speed of both the left and right landinggear may optionally be determined independently. The average wheelspeeds of the left and right landing gear will be compared, and thelesser of the two average wheel speeds will be used to compare with thepredetermined wheel speed threshold. The average wheel speed for eachlanding gear can be calculated independently in this manner so that whenthe airplane is turning and the inboard landing gear wheel speed isbelow the wheel speed threshold, the predetermined minimum residualbrake clamping force will be discontinued.

In this control logic condition, when an aircraft has left and rightlanding gear, the predetermined minimum residual brake clamping forcewill be discontinued if the lesser of the two average wheel speeds isbelow the wheel speed threshold. The average wheel speed for eachlanding gear is calculated independently, so that when the airplane isturning and the inboard landing gear wheel speed is below the wheelspeed threshold, the predetermined minimum residual brake clamping forcewill be discontinued. Disabling the brake drag force below a thresholdwill also ensure that the feature will not interfere with airplanetowing operations, which typically happen at low speed. The brakes willalso be fully released when the airplane is full stop. This will ensurethat the brake drag will not interfere with parking brake operation,when maintenance personnel must replace the wheel/brake, duringbrake-released cooling on the ground, or during system checkout testing.Finally, disabling the brake drag force below a speed threshold willensure that the brakes are released when stowed in the wheel well andprior to touchdown/wheel spinup.

A hysteresis can be incorporated into the wheel speed logic, such thatonce the wheel speed control logic condition has been met and thepredetermined minimum residual brake clamping force has beendiscontinued, the predetermined minimum residual brake clamping forcewould not be applied upon the next commanded initiation of brakingunless the aircraft first reaches a higher ground speed, such as 15knots, for example, but the aircraft would again discontinue thepredetermined minimum residual brake clamping force when the aircraftaverage wheel speed is below a lower speed, such as 2 knots, forexample.

Engine thrust lever position may optionally be monitored to determinethe pilot's intent to accelerate the airplane for takeoff or to begintaxi. An engine thrust lever position monitor 22 detects when any enginethrust lever is in an “advanced” position. If the predetermined minimumresidual brake clamping force has been applied, the predeterminedminimum residual brake clamping force will be discontinued if an enginethrust lever is detected to be in an “advanced” position. Once thrustlevers are not in an “advanced” state, residual brake drag will beenabled after the pilot has subsequently depressed the brake pedal.

When the thrust levers are applied for takeoff, the wheel speedacceleration is quite significant and can be easily detected topositively inhibit any brake drag during takeoff. Therefore, optionally,a wheel speed acceleration monitor 24 can be provided to detectacceleration of the airplane for takeoff or taxiing, and as analternative to monitoring of engine thrust lever position. The brakeactuation controller can compare the wheel speed acceleration with apredetermined acceleration threshold, and application of thepredetermined minimum residual brake clamping force may be discontinuedif wheel speed acceleration beyond the preset acceleration threshold.

The brake temperature monitor system 26 may also be used to providebrake temperature readings to the brake actuation controller to comparewith a predetermined temperature threshold, so that the application ofthe predetermined minimum residual brake clamping force can optionallybe discontinued if the brake temperature increases above the temperaturethreshold. This way the residual brake force will not cause the braketemperature to become too high. Once the brake temperature is above thetemperature threshold, carbon brake wear is already reduced becausecarbon brake wear rates are known to be less at high temperature.

Another optional control logic condition under which the application ofthe predetermined minimum residual brake clamping force could bediscontinued can occur if the distance the aircraft has rolled with apredetermined minimum residual brake drag applied has exceeded adistance threshold. The roll distance traveled can be determined by thebrake actuation controller by using data from the wheel speed monitorand tracking the time since the last brake application command. Once theroll distance has increased above a set threshold, such as two miles,for example, the predetermined minimum residual brake clamping forcewill be discontinued to prevent the brakes from becoming hotter.

Examples of circumstances in which one or more of the control logicconditions should ideally apply to interrupt application ofpredetermined minimum residual brake application clamping force include:during towing and push-back, so that the tow tractor doesn't have tocope with the brake drag; during touchdown/wheel spinup; during antiskidcycling when full dumps are commanded; on the outboard gear during tightturns, since release of the residual drag may be desirable so that thebrakes don't fight the turn; with the landing gear stowed, which may bedesirable for cooling the landing gear in the wheel well; and whenparked with the parking brake released, which also may be desirable forbrake cooling. Typically for such circumstances as touchdown, spinup,and during antiskid cycling, an antiskid system already overridesmetered braking pressure. While for tight turns it may also be desirableto optionally implement a steering control logic condition by monitoringsteering or tiller position, this would normally not be necessary, sincetypically release of the predetermined minimum residual brake clampingforce would already take place when any such tight turns might occur,due to the monitoring of wheel speed as a control logic condition.During turns, the speed of the wheels on the inboard side of the turnwill travel more slowly than those on the outboard side, and thedifferential will become greater as the turn gets tighter. The effect ofthe wheel speed logic would be to remove the “slight drag” virtually anytime the aircraft makes a tight turn, thereby reducing the differentialthrust required to make the turn.

Although it is also possible to optionally monitor stowing of thelanding gear and parking, due to monitoring of wheel speed, release ofthe predetermined minimum residual brake clamping force would normallytake place when the landing gear is stowed or the airplane is parked,due to the control logic that releases the brakes below a wheel speedthreshold. It should also be noted that brake release commands from anantiskid control system always override any brake application command,i.e. a full release from the antiskid control system will always resultin full release of the brake application clamping force.

The result for various phases of operation is as follows:

Parked at the ramp: Brakes will fully release (wheel speed below 2 to 10knots).

Pushback: Brakes will fully release (wheel speed below 2 to 10 knots).

Very slow taxi (below 2 to 10 knots): Brakes will fully release (wheelspeed below 2 to 10 knots).

Normal taxi (above 2 to 10 knots): Brakes will fully release until firstbrake snub, and then brakes will gently “ride.”

Tight turns: Brakes will fully release (tight turns require slow speed,inboard-gear wheel speed below 2 to 10 knots).

Takeoff roll:

-   -   Normal operation: Brakes will fully release (thrust levers        advanced).    -   Abnormal operation: For RTO with sufficient braking to induce        antiskid action, brakes will fully release until 1st brake        application. Then brakes will fully release whenever antiskid        commands it. If antiskid doesn't command full release then        brakes will gently “ride”.

Liftoff: Brakes will fully release (thrust levers advanced).

Gear retract: Brakes apply due to gear retract braking, then fullyrelease when gear retract braking command is removed (wheel speed below2 to 10 knots).

Stowage in wheel well: Brakes will fully release (wheel speed below 2 to10 knots).

Gear extension before touchdown: Brakes will fully release (wheel speedbelow 2 to 10 knots).

Touchdown/spinup (pedals not applied).

Normal operation: Brakes will fully release (brakes not re-applied sincewheel speed below 2 to 10 knots).

Abnormal operation: Touch down/spinup with pedals applied, brakes willfully release (touchdown/hydroplane protection already resident inantiskid).

Landing rollout, either manual or automatic braking (no antiskidaction).

Normal operation: Brakes will fully release until 1st brake application.Then brakes will gently “ride.”

Abnormal operation: Landing rollout with sufficient braking to induceantiskid action, brakes will fully release until 1st brake application.Then brakes will fully release whenever antiskid commands it. Ifantiskid doesn't command full release then brakes will gently “ride.”

Taxi in (above 2 to 10 knots): Brakes will fully release until 1st brakesnub. Then brakes will gently “ride.”

Final maneuvering and docking (below 2 to 10 knots): Brakes will fullyrelease (wheel speed below 2 to 10 knots).

Setting the parking brake, then releasing: Brakes will fully release(wheel speed below 2 to 10 knots).

Operation with hot brakes: Brakes will fully release at all times (hotbrakes per brake temp monitor).

As is illustrated in FIG. 3, the overall brake energy for normal brakingwith multiple brake snubs is substantially equivalent to controllingapplication of aircraft carbon brakes according to the invention, butthe number of taxi brake applications is reduced from five brakeapplications using normal braking, to one braking application by themethod of the invention. The number of taxi brake applications thus canbe substantially reduced by the method of the invention, resulting insignificantly reduced aircraft carbon brake wear.

As is illustrated in FIG. 4, in the method and system of the invention,each individual brake 10 is provided with a plurality of electric brakeactuators 30, consisting of a first portion of electric brake actuators,such as a first pair of electric brake actuators 32 a, 32 b, having afirst range of low brake clamping force responsive to low brake clampingforce commands, and a second portion of electric brake actuators, suchas a second pair of electric brake actuators 34 a, 34 b, having a secondrange of high brake clamping force responsive to high brake clampingforce commands. The first and second pairs of electric brake actuatorsare connected to the brake actuation controller 18, and in a preferredaspect, the first and second pairs of electric brake actuators arearranged in a balanced configuration in the brake, such as with thefirst pair of electric brake actuators 32 a, 32 b placed in radiallyopposing positions in the brake, and the second pair of electric brakeactuators 34 a, 34 b similarly placed in radially opposing positions inthe brake. In a presently preferred aspect, the second pair of electricbrake actuators are placed between the first pair of electric brakeactuators, and the second pair of electric brake actuators are typicallyplaced symmetrically between the first pair of electric brake actuators.

In the method according to the invention, once the residual minimumbrake clamping force is engaged, it is maintained for the plurality ofwheel brakes despite a commanded release of braking, such as byactuation of brake pedal by a pilot, an autobrake system, or gear-upbraking system, for example, of any of the plurality of wheel brakes. Ina preferred aspect, the first pair of electric brake actuators 32 a, 32b, is actuated to maintain the predetermined minimum residual brakeclamping force until one or more control logic conditions occurs, inresponse to which the application of the predetermined minimum residualbrake clamping force is discontinued, and the second pair of electricbrake actuators 34 a, 34 b, is engaged only when the braking force to beapplied falls within the second range of high brake clamping force. Whenthe commanded braking force falls below the second range of high brakeclamping force, the second pair of electric brake actuators isdisengaged. The cumulative range of clamping force of the first andsecond pairs of electric brake actuators is equivalent to that ofcurrent electric brake actuators, but because the entire range isdivided between the first and second portions of electric brakeactuators, and the accuracy of the first portion of low force electricbrake actuators applies over a smaller range, the cumulative brake forceof the first and second portions of electric brake actuators issignificantly improved, particularly at low speeds, when steering of theaircraft is commonly controlled by braking, and particularly when aminimum light residual clamping brake force is maintained during taxiingwhen braking is no longer commanded.

1. A method for controlling electric carbon brakes of an aircraft whenthe aircraft is taxiing, the aircraft having left and right landing gearwith a plurality of wheels, and a corresponding plurality of wheelbrakes, the method comprising the steps of: providing a first portion ofa plurality of electric brake actuators of each wheel brake with a firstrange of low brake clamping force responsive to low brake clamping forcecommands; providing a second portion of the plurality of electric brakeactuators of each brake with a second range of high brake clamping forceresponsive to high brake clamping force commands, said second range ofhigh brake clamping force being greater than said first range of lowbrake clamping force; monitoring commanded initiation of braking of anyof the plurality of wheel brakes of the aircraft; applying apredetermined minimum residual brake clamping force with said firstportion of a plurality of electric brake actuators for the plurality ofwheel brakes in response to said commanded initiation of braking of anyof the plurality of wheel brakes; determining a left side average wheelspeed of the wheels of the left landing gear; determining a right sideaverage wheel speed of the wheels of the right landing gearindependently of the left side average wheel speed; comparing said leftside average wheel speed with a first predetermined wheel speedthreshold; comparing said right side average wheel speed with said firstpredetermined wheel speed threshold; discontinuing applying saidpredetermined minimum residual brake clamping force of said firstportion of a plurality of electric brake actuators when either of saidleft side average wheel speed and right side average wheel speed is lessthan said first predetermined wheel speed threshold; applying saidsecond range of high brake clamping force only when the commandedbraking force is in the second high range of brake clamping force; anddiscontinuing applying said predetermined minimum residual brakeclamping force of said first portion of a plurality of electric brakeactuators when the commanded braking force is less than the second highrange of brake clamping force.
 2. The method of claim 1, wherein theaircraft includes at least one engine thrust lever, and said at leastone second control logic condition comprises said at least one enginethrust lever being in an advanced position.
 3. The method of claim 1,further comprising the step of detecting wheel speed acceleration, andcomparing said wheel speed acceleration with a predeterminedacceleration threshold, and wherein said at least one second controllogic condition comprises said wheel speed acceleration exceeding saidacceleration threshold.
 4. The method of claim 1, further comprisingmonitoring temperature of said plurality of wheel brakes, and comparingsaid temperature of said plurality of wheel brakes with a temperaturethreshold, and wherein said at least one second control logic conditioncomprises said temperature of said plurality of wheel brakes exceedingsaid temperature threshold.
 5. The method of claim 1, further comprisingdetermining distance rolled with said predetermined minimum residualbrake clamping force applied, comparing said distance rolled with apredetermined distance threshold, and wherein said at least one secondcontrol logic condition comprises said distance rolled exceeding saidpredetermined distance threshold.
 6. The method of claim 1, furthercomprising: comparing said left side average wheel speed with a secondpredetermined wheel speed threshold greater than said firstpredetermined wheel speed threshold; comparing said right side averagewheel speed with said second predetermined wheel speed threshold;applying said predetermined minimum residual brake clamping force withsaid first portion of a plurality of electric brake actuators for theplurality of wheel brakes in response to said commanded initiation ofbraking of any of the plurality of wheel brakes when either of said leftside average wheel speed and said right side average wheel speed reachessaid second predetermined wheel speed threshold; and discontinuingapplying said predetermined minimum residual brake clamping force ofsaid first portion of a plurality of electric brake actuators wheneither of said left side average wheel speed and said right side averagewheel speed is less than said first predetermined wheel speed threshold.